Autophagy is a self-eating mechanism characterized by formation of the double-membrane vesicle called autophagosome (AP). Under autophagy induction conditions, targets are enclosed in AP and then transported to vacuoles for degradation.
In yeast, Atg9, a sole transmembrane ATG protein, has lipid scramblase activity and is involved in elongation of pre-autophagosomal structures (isolation membrane; IM). Thus, in atg9 deletion null mutants, IM cannot elongate and thus autophagosome formation is completely abolished. Moreover, in atg9 point mutants having amino acid mutations which cause a reduction of the scramblase activity, autophagosome size decreases compared to wild-type.
Interestingly, however, Arabidopsis atg9 T-DNA insertion null mutants still had autophagic activities although delayed progression of autophagy was detected compared to wild-type. Furthermore, unlike in yeast, the size of autophagic bodies in the atg9 mutants was larger than that in wild-type. By microscopic and biochemical analyses, we found that Arabidopsis ATG9 localizes on very small dot-like structures which are probably related to Golgi compartments. Co-localization analyses of ATG9 with AP or various organelles suggested that plant-specific function of ATG9 might result from ATG9 interacting factors. Additionally, we successfully established a novel method for the isolation of ATG9 vesicles with high-purity using a co-IP technique. Here, we discuss possible roles of ATG9 in plants based on the results of above-mentioned cell biological and biochemical analyses.